1. Field of the Invention
This invention relates to the field of blood oximetry. More particularly it relates to an improved optical blood oximeter which non-invasively measures blood oxygen content by backscattered light.
2. Description of Related Art
1. Theory
The basic concepts of optically based oximetry were described in the early 1930's and expanded in the 1940's in the works of Nicolai [L. Nicolai, Uber Sichtbarmachung, Verlauf and Chemische Kinetik der Oxyhemoglobinreduktion im Lebenden Gewebe, Besonders in der Menschlichen Haut, Arch. Gesamte Physiol. 229: 372 (1932)] and Kramer [K. Kramer, Ein Verfahren zur Fortlaufenden, Messung des Sauerstoffgehalets im Stromenden Blute en Uneroffneten Gefassen, Ziet. f. Bilogie. 96: 61 (1935)] and Matthes [K. Matthes, Untersuchungen uber die Sauerstoffsaptingung des Menschlichen Arterienblutes, Arch. f. Exper. Path, U Pharmakel. 179: 698 (1935)] respectively. These early methodologies were based upon measuring the difference in the optical transmission spectrum of oxygenated and deoxygenated hemoglobin. The measurements were made at the isobestic point of hemoglobin and the point of maximum transmission (HbO.sub.max). HbO.sub.max is that frequency which is maximally affected by changes in the oxygen content of hemoglobin. It is typically 660 nanometers ("nm"). By knowing the relative intensities of red and infrared sources, it was possible to use the Lambert-Beer law to determine the relative amount of hemoglobin in solution and its oxygenation state. A complete derivation of the theory is presented below:
The Lambert-Beer law is stated as: EQU I(t)=I(o)exp.sup.(-acd) (Equation 1)
where:
I(t)=Intensity of transmitted light PA1 I(o)=Source intensity PA1 a=molar absorption coefficient (sample) PA1 c=concentration (sample) PA1 d=optical path length PA1 OS=Oxygen saturation PA1 A=Constant (experimentally determined) PA1 B=Constant (experimentally determined) PA1 OD1=Optical density at HbO.sub.max (lambda red) PA1 OD2=Optical density at isobestic point (lambda IR) PA1 C=Constant (experimentally determined) PA1 D=Constant (experimentally determined) PA1 1. Probe--contains the light sources and detector diode; PA1 2. Amplifier--used to magnify the signal received (via electric cable) from the probe; PA1 3. Filters--used to remove extraneous noise and to compensate for movement artifact; PA1 4. Power Supply--provides power to the probe, amplifiers, processor electronics, and display electronics; PA1 5. Processor electronics--convert the analog signal (from the probe) into a digital signal for processing by various algorithms. Usually consists of a micro computer and various proprietary software tools; and PA1 6. Display Electronics--Cathode ray tube and electronics necessary to display the information to the user. PA1 a. optical path differences due to probe position/tissue volume changes; PA1 b. path differences due to motion of probe relative to tissue; PA1 c. differences in spectral characteristics of LEDs of the same type makes repeatability of readings from machine to machine difficult; PA1 d. divergence from optical density versus lambda curves due to wide bandwidth of LEDs leads to erroneous saturation determinations; and PA1 e. optical misalignment of sensor vs. detector makes sensitivity to hematocrit concentration critical. PA1 a. depth of penetration and subsequent "blurring" of the intensity curves and refractive index shifts at connective tissue interfaces; PA1 b. size of invasive devices makes them impractical; and PA1 c. inability to resolve the source of the backscattered light leads to processing errors and deviations in values of Oxygen saturation measured by a pulse oximeter versus invasive measurement techniques. PA1 "Although the pulse oximeter has achieved the status of routine use in many clinical settings, there are several areas where research on improvements of the technique is under way or needs to be encouraged. Noninvasive tissue pulse oximeters in use today operate in the transmission mode, and so the number of tissue sites where the instrument can be applied are limited to structures that can be easily transilluminated such as the fingers or toes. These locations are often not the most desirable due to their being at the extreme periphery of the circulatory system and therefore first to be compromised in circulatory shock. These locations are also likely to encounter more motion, and hence, more motion artifact than more central positions on the chest or abdomen. Historically it has been possible to look at in vitro oximetry from the backscatter (reflection) mode as well as the transmission. This step, however, has not been possible in practical noninvasive tissue oximeters. Research in understanding backscattered light from illuminated skin and subcutaneous tissue is necessary before practical instruments can be developed. Since light can penetrate deep in the body, one must be concerned about this depth of penetration, the effects of reflecting from subdermal tissue interfaces, and the sensitivity to local anatomy, and hence, the position of the sensor in such instruments." PA1 "The availability of a practical backscatter oximeter would be an important development in that more locations would become available for clinical noninvasive oximetry. It would be possible to develop an intrapartum fetal pulse oximeter that could be used to monitor hemoglobin oxygen saturation in the fetal presenting part during labor and delivery. Although some investigators have made simultaneous recordings form transcutaneous oxygen sensors and pulse oximeters on the same subjects, the sensors had to be located at different sites due to their different location requirements. A backscatter mode pulse oximeter would make it practical to combine a transcutaneous PO.sub.2 sensor and pulse oximeter into the same structure so that simultaneous measurements of oxygen tension and saturation could be made at the same site." PA1 1. The new device shall be non-invasive; PA1 2. Saturation readings taken with the device shall be independent of probe placement given a minimum tissue depth of approximately 2-3 millimeters; PA1 3. The new device must account for or obviate the problems caused by index shifts at connective tissue interfaces; PA1 4. The device should have improved repeatability and improved accuracy; and PA1 5. The device should avoid burning tissue through long term exposure to high intensity light.
If the Lambert-Beer law is restated as a function of optical density (OD), equation (1) becomes: EQU OD=log(20)I(o)/I(t)=acd (Equation 2)
If the path length is held constant and measurements are carried out at widely spaced wavelengths, then equation (2) becomes: EQU OS=A-B(OD1/OD2) (Equation 3)
where:
In 1935, an ear oximetry device was described [K. Matthes, Untersuchungen uber die Sauerstoffsaptingung des Menschlichen Arterienblutes, Arch. f. Exper. Path, U Pharmakel. 179: 698 (1935)] that employed the transmission theory described by Equation (3). The device was prone to errors due to skin pigmentation and the optical properties of whole blood. Also, for the data to be valid, the skin had to be heated to approximate arterial blood.
If we examine what happens to light entering a hemoglobin solution we find that it has three components: transmission, absorption, and reflection. Application of the Lambert-Beer law for the case of reflection and application of the principles used in the derivation of equation (3) yield : EQU OS=C-D(X2/X1) (Equation 4)
where:
X1=Optical density at HbO.sub.max (lambda red)
X2=Optical density at isobestic point (lambda IR)
In 1944, Drabkin [D. L. Drabkin and C. F. Schmidt, Spectrophotometric Studies XII. Observation of Circulating Blood In Vitro and the Direct Determination of the Saturation of Hemoglobin in Arterial Blood, J. Biol. Chem. 157: 69 (1944)] demonstrated an invasive device that gave very good results when compared against gasometric analyses. A non-invasive device was demonstrated in 1949 [R. Brinkman, W. G. Zijistra and R. K. Koopmans, A Method for Continuous Observation of Percentage Oxygen Saturation in Patients Arch. Chir. Neeri. 1:333 (1949 )] that also correlated well with gasometric analyses, but suffered from problems in repeatability across pigmentation and tissue types. In 1980 a multispectral device was developed [S. Takatani, P. W. Cheung and E. A. Ernst, A Noninvasive Tissue Reflectance Oximeter, Ann. Biomed. Engrg. 8: 1 (1980)] that addressed the pigmentation problem but still suffered from other problems related to path length resolution.
A new measurement technique was introduced in 1980 for the determination of oxygen saturation [I. Yoshiya, Y. Shimada and K. Tanaka, Spectrophotometric Monitoring of Arterial Oxygen Saturation in the Fingertip, Med. Biol. Engrg. & Comput. 18: 27 (1980)]. The measurement looks at the variation of optical density as a function of variations in blood volume during the cardiac cycle, hence the name pulse oximetry. The unique feature of this device is its ability to look at arterial blood without the necessity of heating the skin. This ability arises because the calculations are based on capillary blood volume variations resulting during the cardiac cycle. During systole, a fresh bolus of blood infuses the capillary bed increasing the blood volume and hence the optical density. During diastole, the venous runoff will decrease the volume with an attendant decrease in optical density. Since the optical density change at the isobestic point is the same from pulse to pulse and the optical density will change at HbO.sub.max with variations in oxygen saturation, equation (3) expresses the transmission case and equation (4) the reflection case.
2. Technology
According to Neuman [Michael R. Neuman, Pulse Oximetry: Physical Principles, Technical Realization and Present Limitations], commercially available non-invasive devices consist primarily of the transmission type. They usually consist of the following components:
FIG. 1 depicts the arrangement of these components in the majority of commercially available prior art devices. The components depicted are common to both transmissive and reflection devices. FIGS. 2A, 2B and 2C depict the arrangement of the probes of several prior art transmissive devices. FIG. 3 depicts a prior art reflection-type blood oximeter sensor for use in the oesophageal airway.
3. Operation
Most commercially available pulse oximeters are microprocessor controlled. Software in the computer directs the sequencing of events such as turning on the various light sources and then reading the transmitted intensity of either infrared or red light. Once the intensities are known, the microprocessor is then tasked with performing several table look ups and fitting the data to the following equation: EQU SO.sub.2 =a(A*LAMBDA1/A*LAMBDA2)+b (Equation 5)
where: EQU a=extLAMBDA2Hb/(extLAMBDA2Hb-extLAMBDA1HbO) (Equation 6)
and EQU b=extLAMBDA1Hb/(extLAMBDA1Hb-extLAMBDA1HbO) (Equation 7)
The extinction coefficients are experimentally determined parameters that are dependent upon the particular devices (LEDs), wavelengths, and half-intensity bandwidths of the narrowband light sources. Processing for the values of a and b are the source of most of the table look-ups.
Operational amplifiers A, B, C, and D are used to amplify the signals coming from the two channels of the digital to analog converter (component J of FIG. 1). Amplifiers C and D are also used to adjust the cut-off point of the LEDs via the fine adjustment of resistors R1 and R2 respectively.
When either of the LEDs is on, the light passes through the sample and is measured by the PIN diode (component G of FIG. 1) and is then translated into a digital signal via the analog to digital converter (component L of FIG. 1). When the translation is completed, the resultant number is stored in the computer for later processing.
Besides processing the plethysmographic signals, the microprocessor is also responsible for the processing required to remove movement artifacts, temperature drifts, and ambient light noise signals. FIG. 4 depicts how the noise and artifact signals combine to affect the plethysmographic signal.
4. Problems
Transmissive devices require the capability to transilluminate a group of vessels and are thereby restricted to locations on the periphery of circulation. The measurement is susceptible to errors when using vasoconstrictors or when a patient is in shock, i.e., poor peripheral perfusion. There are other sources of error:
Reflective devices also suffer from several problems:
There are mechanical problems associated with prior art devices. In instances where perfusion is poor or in cases involving neonates, the temperature rise associated with contact with the LEDs has led to cases of first and second degree burns [J. Bannister, D. H. T. Scott, Thermal Injury Associated With Pulse Oximetry, Anaesthesia.].
Neuman [Michael R. Neuman, Pulse Oximetry: Physical Principles, Technical Realization and Present Limitations] established the need for an improved pulse oximeter when he wrote the following:
Having established the need for a backscatter device versus a merely improved transmissive device, it would be desirable for the new device to have the following attributes:
Satisfaction of requirements 1 and 2 rely on the development of a completely new approach to the measurement of the backscattered light intensity. One of the fundamental problems with backscattered light measurements and transmitted light measurements has been knowing (exactly) the properties of the reflecting or absorbing medium. To make the measurement process as independent of the physical properties of the absorbing/reflecting medium as possible requires extending the work of Twersky [Victor Twersky, Multiple Scattering of Waves and Optical Phenomena, Journal of the Optical Society of America. 52: 2 (1962)]. In his paper on the "Multiple Scattering of Waves and Optical Phenomena" he presents the general form for the phase change of the coherently reflected wave as: ##EQU1## which is equal to 180 degrees whether the incident beam is parallel or perpendicularly polarized with respect to the incident plane. The coherent power reflection coefficient is stated by Twersky as: EQU R=.vertline.(1+Z)/(1-Z).vertline..sup.2
FIG. 5 plots the coherent power reflection coefficient versus the angle of incidence with respect to the normal for parallel and perpendicularly polarized beams. We can see that as the parallel beam angle of incidence approaches 90 degrees the value of R approaches unity as does the perpendicular power reflection coefficient. The implication of equations 8 and 9 is that there should be an increase in sensitivity as a function of "grazing" angle, i.e., distance from source to receiver. This implication has been experimentally verified by Schmitt et al [J. M. Schmitt, F. G. Mihm, J. D. Meindl, New Methods For Whole Blood Oximetry, Annals of Biomedical Engineering, 14: 35 (1986)]. In the same study, it was also shown that the oxygen saturation measurement became more linear the closer the source was placed to the detector. The study also showed that as the source and detector separation increased there was a marked change in intensity (reflected power) as a function of hematocrit level.
The change in sensitivity as a function of source/receiver separation is due to the fraction of coherently reflected light into the forward versus backward half spaces [Victor Twersky, Multiple Scattering of Waves and Optical Phenomena, Journal of the Optical Society of America. 52: 2 (1962)]. LEDs, being essentially incoherent and non-polarized, will generate more reflected light into the forward half space than into the back half space. Therefore, as the grazing angle is reached they will appear to become more sensitive until the separation of the source and receiver causes the receiver's half angle to be exceeded and then no power is received.
The invention herein overcomes the drawbacks of the prior art by using a LASER as the source (polarized parallel to the incident plane) and a single mode monofilament light guide to deliver the light and gather the reflected light. By utilizing this system, the mathematical description of the system appears as if the source and detector are collocated. The byproduct of this apparent collocation is increased linearity and the coherently reflected fraction of the light approaches unity. The reason one polarizes the light parallel to the incident plane is to eliminate the sensitivity to hematocrit concentration which has been demonstrated by prior art reflection devices [J. M. Schmitt, F. G. Mihm, J. D. Meindl, New Methods For Whole Blood Oximetry, Annals of Biomedical Engineering, 14: 35 (1986)].